Many power supplies and electronic ballast for gas discharge lamps employ bridge-type inverters. Bridge-type inverters include full-bridge inverters (which require four power switching devices) and half-bridge inverters (which require two power switching devices). The power switching devices are commonly realized by power transistors, such as bipolar junction transistors (BJTs) or field-effect transistors (FETs). Bridge-type inverters are generally classified into two groups—self-oscillating or driven—according to the approach that is used to provide commutation (i.e., switching on and off) of the power transistors.
Driven bridge-type inverters employ a dedicated driver circuit for commutating the inverter transistors, and typically include a current feedback arrangement (coupled between the inverter transistors and the driver circuit) to ensure that the peak current that flows through the inverter transistors during conduction is maintained within a reasonable limit. This is necessary in order to prevent excessive heating of the transistors (due to conduction power losses) and, more fundamentally, to ensure that the maximum current ratings of the transistors are not exceeded. In any case, the current feedback arrangement is required in order to preserve the long-term reliability of the inverter and to protect the inverter transistors from destruction.
A common current feedback arrangement employs a current sensing resistor (e.g., resistor 150 in FIG. 1) that is connected in series with one or more of the inverter transistors (e.g., transistor 120 in FIG. 1). The instantaneous voltage that appears across the current sensing resistor is proportional to the instantaneous current that flows through the inverter transistors during conduction. The voltage across the current sensing resistor is provided to a current-sense input (e.g., input 132 in FIG. 1) of the driver circuit (e.g., driver circuit 130 in FIG. 1) by way of an RC filtering network (e.g., resistor 154 and capacitor 156 in FIG. 1) that substantially prevents high frequency noise or other spurious signals from reaching the current-sense input of the driver circuit. During operation, the driver circuit continuously monitors the voltage at the current-sense input. If the voltage at the current sense input attempts to exceed a predetermined limit (e.g., about 0.6 volts or so, but sometimes variable based on mode of operation), the driver circuit modifies the nature of the drive signals (e.g., which are provided via drive outputs 140,144 in FIG. 1) for the inverter transistors (e.g., transistors 110,120 in FIG. 1) in such a way as to reduce the instantaneous current flow through the inverter transistors.
The aforementioned approach usually performs well under many types of operating conditions. However, it has been found that this approach does not effectively limit the instantaneous current flow through the inverter transistors under certain conditions, such as when the inverter operates in a so-called “capacitive switching” mode. For those applications in which an inverter drives a resonant output circuit (e.g., inductor 210 and capacitor 212 in FIG. 1), which is a common in arrangement in many types of electronic ballasts and in some types of power supplies, capacitive-mode switching tends to occur when the direct current (DC) input voltage supplied to the inverter (e.g., VRAIL in FIG. 1) falls significantly below its normal value (i.e., when VRAIL “falls out of regulation”).
During capacitive-mode switching, the current that flows through the inverter transistors (during conduction) is generally characterized as having a high amplitude (i.e., a high peak value) and a short duration (i.e., a narrow pulse width). At the same time, the DC input voltage VRAIL has been significantly reduced (which is what gave rise to capacitive-mode switching in the first place). Under those conditions, during which the peak value of the current through the inverter switches may reach many multiples (e.g., 8 to 25 times) of its normal operating value, the conventional approach (i.e., utilizing a current sensing resistor and an RC filter coupled to the current-sense input of the driver circuit) does not effectively limit the peak value of the current through the inverter transistors. Consequently, the inverter transistors experience a level of power dissipation (and associated heating) that may cause the junction temperatures within the transistors to become so high that the transistors are destroyed.
Thus, a need exists for an inverter having a protection circuit that is capable of reliably protecting the inverter transistors from overcurrent conditions that occur during capacitive-mode operation. Such an inverter, as well as a power supply or an electronic ballast that includes such an inverter, would represent a considerable advance over the prior art.